The measuring apparatus includes a magnet arrangement, which produces an alternating magnetic field passing through a measuring tube essentially perpendicularly to the measuring tube axis; two measuring electrodes positioned in the measuring tube on a connecting line directed essentially perpendicularly to the measuring tube axis and to the magnetic field; optionally, at least one reference electrode; and a control/evaluation unit, which delivers information concerning the volume- or mass-flow of the medium in the measuring tube on the basis of the measurement voltage induced in the measuring electrodes and which determines electrical conductivity of the medium on the basis of impedance measured between two electrodes.
Magneto-inductive flow measuring devices, the essential components of which have been set-forth above, make use of the principle of electrodynamic induction for measuring volumetric flow: Charge carriers of the medium moved perpendicularly to a magnetic field induce a voltage in measuring electrodes likewise arranged essentially perpendicularly to the flow direction. This measurement voltage induced in the measuring electrodes is proportional to the flow velocity of the medium averaged over the cross section of the measuring tube; it is, thus, proportional to volume flow rate. In the case of known density of the medium, it is, moreover, possible to derive also the mass flow rate of the medium flowing through the measuring tube. The measuring electrodes are usually galvanically or capacitively coupled with the medium.
Ordinarily, control of the alternating magnetic field is accomplished using the coil current: In the ideal case, the magnetic field is proportional to the electrical current flowing in the coil, or coils, of the magnet arrangement, since the measuring- or field-frequency of the magneto-inductive flow measuring device (thus, the frequency, with which reversal of the alternating magnetic field occurs) is dependent, in high degree, on the inductance of the coil arrangement.
From a number of publications, it has become known, in the case of a magneto-inductive flow measuring device, to generate, besides information concerning flow, also information concerning other state variables of the medium or the measuring system. Thus, it is known from US2003/0051557 A1 to apply a diagnostic signal between the measuring- and/or reference-electrodes and to obtain information concerning the electrical conductivity of the medium on the basis of resistance values of the diagnostic signals.
From JP-A-174718 and EP 0 336 615 B1, it has likewise become known to determine electrical conductivity of the medium flowing through the measuring tube via resistance values measured between two electrodes. In the case of JP-A-174718, the obtained information concerning the conductivity of the medium is used for correction of the measured values of flow. EP 0 336 615 B1 provides a solution which enables measured values regarding flow and conductivity to be provided simultaneously.